Tuning the Ground State and Excited State Properties of Monocationic Iridium(III) Complexes by Varying the Site of Benzannulation on Diimine Ligand.

Extending π-conjugation of the diimine ligand (N^N ligand) via benzannulation is a common way to tune the absorption and emission energies of cationic iridium(III) complexes. However, it can cause either a red- or blue-shift of the absorption and emission bands depending on the site of benzannulation. To understand the mechanism of changes in optical transitions upon benzannulation on the diimine ligand, a series of new cationic iridium(III) complexes [Ir(dppi)2(N^N)]PF6 (1-6) (where dppi =1,2-diphenylpyreno[4,5-d]imidazole; N^N = 2-(pyridin-2-yl)quinoline (1), 2-(pyridin-2-yl)[7,8]benzoquinoline (2), 2,2'-bisquinoline (3), 2-(quinolin-2-yl)[7,8]benzoquinoline (4), 2-(pyridin-2-yl)[6,7]benzoquinoline (5), 2-(quinolin-2-yl)[6,7]benzoquinoline (6)) containing diimine ligand with varied degrees of π-conjugation via benzannulation at different sites of the 2-(pyridin-2-yl)quinoline ligand were synthesized. Experimental results and density functional theory (DFT) calculations revealed that benzannulation at the 6,7-position of quinoline and/or the 5',6'-position of pyridine (3, 5, and 6) induced red-shifts in their absorption and emission bands with respect to the parent complex 1; while benzannulation at the 7,8-position of quinoline resulted in blue-shifts (2 vs 1 and 4 vs 3). This phenomenon was rationalized by the symmetry of the frontier molecular orbitals at the site of benzannulation, which stabilized or destabilized the lowest unoccupied molecular orbital (LUMO) upon interactions with 1,3-butadiene, while the energy of the highest occupied molecular orbital (HOMO) remained nearly the same. This discovery would enable a rational design of organic or organometallic compounds that have predetermined absorption and emission energies.